1. Field of the Invention
The present invention relates to an improvement of a thermal mass flow meter provided with a sensor portion formed by winding two sensor coils around an outer circumference of a conduit, through which a fluid passes, at a suitable interval.
2. Description of the Prior Art
In the conventional thermal mass flow meter, in order to reduce a posture error resulting from the convection, various kinds of device have been made.
That is to say, a device, in which two sensor coils 41, 42 wound around an outer circumference of a metallic conduit 40, through which a fluid passes, at a suitable interval and a conduit portion, around which said sensor coils 41, 42 are wound, are covered with an insulating cladding material 43 made of a material having a low heat conductivity and an insulating property, as shown in FIG. 4 (refer to Japanese Patent Publication No. Sho 56-23094), a device, in which the greater part of a metallic conduit 50, through which a fluid passes, two sensor coils 51, 52 wound around an outer circumference of said conduit 50 at a suitable interval are provided within a groove 54 having an equivalent diameter of 4 mm or less formed in a case (for example a metallic case) having a heat conductivity remarkably larger than that of air, as shown in FIG. 5 (refer to Japanese Patent Application Laid-Open No. Sho 59-88622), a device, in which both end portions of a metallic conduit 60, through which a fluid passes, and around of which outer circumference two sensor coils 61, 62 are wound at a suitable interval, are joined together by means of a belt-shaped film 63 superior in heat conductance to make said both end portions equal to each other in temperature, as shown in FIG. 6 (refer to U.S. Pat. No. 4,686,856), and the like have been known.
In addition, referring to FIG. 4, reference numerals 44, 45 designate bridge resistances composing a bridge circuit 46 together with said sensor coils 41, 42 and reference numerals 47, 48 designate output terminals. And, said bridge circuit as shown in FIG. 4 is formed also in FIG. 5 and FIG. 6.
However, in means shown in FIG. 4, a disadvantage has occurred in that a responsiveness is inferior since a heat capacity of said insulating cladding material 43 is large. In addition, in means shown in FIG. 5, a responsiveness can be improved to some extent since a heat radiated from the conduit 50 can be absorbed to some extent by said metallic case 53 but a response time-constant is 2 seconds, which is large, and said responsiveness is missing as well. And, in means shown in FIG. 6, although the responsiveness can be considerably improved in comparison with the former two means, a disadvantage occurs in that a heat from said sensor coil 61 is taken away by said film 63 and a heat exchange of the coil.fwdarw.a conduit wall.fwdarw.said fluid, which should properly occur, can not be smoothly conducted, whereby an output of a bridge circuit can not be increased in spite of an increase of a flow rate of the fluid to spoil a linearity between an output signal and said flow rate.